At least 18 people have been killed and more than a dozen injured following the collapse of a bridge which triggered a stampede during a Hindu festival in the eastern Indian city of Patna.

"Bodies of the 18 people killed in the stampede have been sent to the hospital for autopsies," Jayant Kant, a police superintendent in Patna told AFP.

Mr Kant said the stampede occurred when a makeshift bridge erected to help people reach the Ganges river gave way under the weight of devotees rushing to offer prayers to the setting sun as part of an annual Hindu religious ritual.

Most of the casualties are thought to be the result of the stampede and not the collapse of the low-slung bamboo-and-rope bridge designed to help worshippers cross rough terrain.

"Ten women and eight children are among those killed," the police officer said, adding the toll was likely to go up as several other Hindu devotees were reported missing at the site.

Television stations showed ambulances with sirens wailing ferry worshippers to various city hospitals.

Sanjay Kumar Singh, a city administrator, said power darkness at the site made rescue efforts more difficult.

"When the bridge collapsed, power cables strung on it snapped and lights went off and in the darkness people scrambled which triggered the stampede," he said.

Patna is capital of the eastern Indian state of Bihar, where the annual Chhath festival dedicated to the Hindu Sun God is popular.

An estimated 400,000 Hindu devotees thronged up to 65 riverside locations specially prepared by state authorities to cater to worshippers travelling to the Ganges, which is revered by Hindus as holy.

Around 50,000 people were present at Adalat Ganj, one of the worship locations, when the makeshift bridge collapsed, officials said.

The festival is celebrated across India and the number of devotees are likely to swell on Tuesday dawn when worshippers will throng rivers to offer prayers to the rising sun.